Though you think you’re using the most accurate water-test kit available, it still may read incorrectly or differently than expected. To save yourself confusion, time and money, you need to understand the test equipment you’re using and what causes it to sometimes report false readings. 

Here’s a look at the top causes of inaccurate readings for liquid kits, test strips and electronic devices.

Liquid kits

Most liquid test kits use a chemical indicator called DPD to measure free chlorine. However, high levels of combined chlorine (chloramines) can cause false positives with these kits.

Low to moderate levels of combined chlorine are neutralized in most DPD kits. But combined chlorine occasionally will build up to such a point that it seems to indicate a level of free chlorine is present when, in fact, there may be none.

A strong chloramine odor may confirm that there’s a high level of combined chlorine present. Otherwise, pay close attention to the chlorine reaction when adding the DPD indicator (usually DPD No. 2). Free chlorine reacts immediately with DPD to turn pink, while combined chlorine generally takes longer to produce a result. Therefore, if color develops within a few seconds or minutes, you are likely seeing combined chlorine reacting with the DPD indicator.

Elevated sanitizer levels can cause problems for liquid test kits, too. First, high levels of chlorine (greater than 15 ppm) or bromine (greater than 20 ppm) can cause the pH indicator to turn purple, which does not match the pH color scale. This is similar to the high pH colors, 8.0 to 8.4 ppm, so be careful not to mistake it for a high reading and then add acid when it’s not necessary.

If chlorine or bromine levels appear to be on the high side and the pH is reading purple, the best thing to do is wait for the sanitizer level to drop and test again. If you’re pressed for time, you could add a drop of sodium thiosulfate to neutralize the chlorine. This is a less than ideal practice because thiosulfate solutions have a high pH. Thus, you are increasing the pH of the sample and again getting potentially high results.

Another problem caused by high sanitizer levels is when the DPD indicator is “bleached out.” A flash of color occurs when the indicator is added and then quickly becomes colorless. If you see this happening, dilute the sample and retest. For example, you may use half sample water from the pool and half fresh water — ideally, distilled water because it contains no chlorine, and little or no chlorine demand.

You can then run the test again using the same procedure. This will provide you with a result that is half the actual value, so you will need to multiply it by 2 to get the correct reading. Higher sanitizer levels may require greater dilution to get an accurate reading.

Tablet test kits

Tablet test kits are another method of testing pool and spa water. These kits are similar to liquid kits, and many of the chemical reactions are the same. Therefore, you can expect to get inaccuracies similar to what is reported in the liquid section.

Additionally, you can usually determine if a tablet is unusable by inspecting the tablet before using it. If a tablet appears to be more powder than tablet, or if it is discolored, it usually indicates that the reagent within the tablet has been contaminated and is no longer usable.

Test strips

Similar to DPD kits, elevated sanitizer levels also can cause problems for test strips. Because test strips and liquids use the same pH indicator — phenol red — high levels of chlorine (greater than 15 ppm) or bromine (greater than 20 ppm) can cause the pH indicator to turn purple, which does not match the pH color scale. This problem tends to be more obvious with test strips, forming a shade of purple that does not closely resemble the 8.4 or other high-end color blocks.

Total alkalinity may be affected as well. Like the pH reading, elevated sanitizer levels can cause a color that does not match anything on the scale, in this case, a royal blue. Generally, the royal blue alkalinity accompanies the purple pH, making it easier to determine when this problem is occurring. The best thing to do is wait for the sanitizer to drop to an acceptable level and test again. 

Another common issue for test strips is faded or “washed out” results. You can identify this situation when test strips react to form colors that appear to be in the same family as the ones on the chart, but are significantly lighter or less intense. This happens most commonly when test strips have been contaminated, generally by heat or humidity.

Test strips should be stored at room temperature, and use dry fingers when handling them to prevent moisture from entering the bottle. Seal the bottle immediately after use.

Electronic devices

Electronic devices don’t provide the same kind of indicators as liquid kits and test strips to let you know when you’re getting inaccurate readings. When using them, you need to be aware when false readings occur.

There are two things to consider with most electronic devices: First, are they calibrated correctly? Most units require a single-point calibration. This means you have a single solution of a known concentration by which to set your meter, thus ensuring it is reading accurately. Some devices require a multiple-point calibration with two or three solutions. If you’re getting questionable readings with your meter, calibrate it to be sure. Some units are factory calibrated and don’t require periodic adjustments.

Second, determine if the device and its accessories are clean. For example, test tubes or vials used with a handheld colorimeter might become discolored or faded over time. Recalibrating can help, but it may not prevent this from causing false high and/or low readings.

Faded or discolored vials or tubes should be replaced, especially if the coloration is not uniform across the entire surface. Cleaning the device can help, but be sure to follow manufacturer’s recommendations. If not done properly, it can damage the device. Regardless of which type of water-test kit you’re using, you can still rely on a proven method to verify the accuracy of a questionable result. Double-check the readings you’re getting against another type of test to confirm that they are in the same general ballpark. If one method tells you the chlorine is low while the other shows that it’s acceptable, one of them is not reading accurately.